Can someone tell me why i can't?

why can't i gain any fat or w.e it is?!?! okay, when you look at somebodies arms there's obviously something there besides muscle that makes them sorta thick, but not for me my arms are bone and so are my legs, im 16 years old about 5'6" and the last 5 or 6 years I've stayed between the weight limit of 108 ~ 123 ive never been over 123, but yet i eat alot, but no matter what i cant gain weight to go past 123, and im tired of having 'skinny' arms, im sort of active well i have been this past year and a half ive been playing tennis, but mostly i just sit on my hind-end and play xboxlive, so why can't i get some girth on my arms (i don't mean around the biceps triceps only i mean my entire arm from the wrist to the shoulder, or should i even worry about it, i think it kinda makes me look anorexic but i eat like a wild pig that hasn't had food for 3 years and is starving with 6 dead deer in front of it, (crappy analogyy i know.) but ive started feeling like my arms are constricted or w.e some times its a slight annoying little pain when i use my arms quite a bit, im thinking its because of the lack of 'fat' on my arms